1. Field of the Invention
The invention relates to an electricity storage system capable of charging an electricity storage unit.
2. Description of Related Art
Japanese Patent Application Publication No. 08-154307 (JP 08-154307 A) describes charging a battery to a full state of charge (SOC) using an external charger. Here, during the external charging, a shift to constant voltage charging is performed while monitoring a charging current and a terminal voltage of the battery, and when charging has been performed for a predetermined amount of time following a reduction in the charging current, the battery is determined to have reached the full SOC.
While the external charging is underway, a voltage value (a closed circuit voltage (CCV)) of the battery increases above an open circuit voltage (OCV) in accordance with an internal resistance of the battery. When the external charging is completed, the CCV of the battery decreases so as to approach the OCV. An amount of voltage drop occurring at the end of the external charging depends on the internal resistance of the battery. As deterioration of the battery progresses, the internal resistance of the battery increases, and therefore, depending on the voltage drop, it may become difficult to charge the battery to the full SOC.